Atomic-scale mechanism for the activation of catalyst surfaces

The hydrogenation and the isomerization of olefins on MoS2 proceed via alkyl intermediates but are independent catalysis, which is inconsonant to the Horiuti-Polyani mechanism for the isomerization and hydrogenation reactions. Detailed studies proved that the hydrogenation reaction proceeds on di-hydride (MoH2) sites and the isomerization reaction takes place on mono-hydride (MoH) sites although both sites are formed on the edge surface of MoS2 in the presence H-2. More dramatic chemical activation was shown with an inactive MoOx surface, where the formation of Mo-alkylidene sites the changes MoOx inert surface a super-active olefin metathesis catalyst. Chemical activation is also recognised on well defined surfaces. Catalytic reaction of NO + H-2 --> 1/2 N-2 + H2O is highly structure sensitive on Pt(100), Pt(110), Rh(100), and Rh(110) surfaces while the reaction is entirely structure insensitive on such bimetallic surfaces as Pt/Rh(100), Rh/Pt(100), Pt/Rh(110), and Rh/Pt(110). It was deduced that formation of active sites having a common local structure is responsible for the structure sensitive catalysis. Complexity of the activation process of bimetallic surface is shown in atomic-scale on a Cu/Pd(111) surface by STM. Base on these results, we could conclude that the optimized catalysts are structure insensitive because their surface will preserve the highest density sites. (C) 1999 Elsevier Science B.V. All rights reserved.